This invention relates generally to continuous wave (CW) radar systems and more particularly to the isolation between transmit and receive antennas used in such systems.
As is known in the art, continuous wave radar systems have a wide variety of applications. In one such system, the transmitting antenna and the receiving antenna are mounted adjacent one another on a common platform. Both the transmitting antenna and the receiving antenna have a plurality of horizontal rows of dipole antenna elements. The antenna elements of each row are coupled, in-phase, to a common feed port through a corporate feed network. Thus, each row of antenna elements produces an antenna pattern having a narrow main lobe, at the center of the row, directed into free space perpendicular to the row. A phase shifting network connects the feed ports of the transmitting antenna to the transmitter, and a phase shifting network connects the feed ports of the receiving antenna to the receiver. The relative phase shifts in the phase shifting networks enables the transmitting antenna and the receiving antenna to point their main lobe antenna patterns at the same desired elevation angle. Thus, the rotation of the transmitting and receiving antennas together as a unit about a vertical axis provides an azimuthal scan. Sequentially changing the relative phase shift provided by the phase shifting networks provides an elevation scan. During the scan, radio frequency energy is transmitted by the transmitting antenna and directed into free space. If the transmitted energy encounters a radar energy reflecting target, a portion of the reflected energy is received by the receiving antenna. The detection of such received energy indicates the presence, or acquisition of, a potential target. It is noted that during operation, the radar transmitter and receiver are on simultaneously for a continuous wave radar system.
For proper operation, it is highly desirable to prevent, or at least reduce to a minimum, the amount of transmitted energy passing directly from the transmitting antenna to the receiving antenna. Excessive power causes an increase in receiver noise above thermal noise and may require additional attenuation in the receiver, limiting the radar's receiver dynamic range. One technique used to provide this isolation is to insert a shield, or septum, between the two antennas. The septum is a planar conductive sheet, positioned in the horizontal plane between the transmitting and receiving antenna elements. Thus, if the sheet were of long enough, transmitted energy would be prevented from passing directly to the receiving antenna. However, as the length of the sheet is increased, the maximum elevation angle is correspondingly reduced. Thus, the system uses a shield of having a length limited by the maximum required elevation angle.